The present invention relates to a harmonic gear drive, which is compact in size and light-weighted, and which can realize high positional accuracy and high gear reduction ratios. More particularly, the present invention relates to a harmonic gear drive, which is suitable for applications in the field of automated machinery such as an articulation driving portion for a robot where silence and smoothness of driving are required, a semiconductor manufacturing device, and reduction gears for a precision actuator, and in the field of aerospace technology such as a control mechanism and a driving mechanism for aerospace machines.
Harmonic gear drive is a mechanical reduction gear device for the transmission of power, which is light and compact and can realize high positional accuracy and high gear reduction ratios without employing any complicated mechanism and structure and with free of backlash. The harmonic gear drive is generally known as a mechanical reduction gear device including a flexspline (flexible gear).
Typically, the conventional harmonic gear drive consists of a rigid circular outer spline with internal teeth, a flexspline with external teeth partly meshed with the internal teeth of the circular spline in a flexible meshing manner, and a wave generator for displacing the meshing points of those internal teeth and external teeth in the circumferential direction for the different number of teeth by means of elastic flexibility. This type of conventional harmonic gear drive is disclosed, for example, in Japanese Laid-open Patent Application No. 10-110790 (paragraphs [0002] to [0003]; FIGS. 1 and 2), which is also referred to as Patent reference 1.
As seen in FIGS. 8 and 9, the harmonic gear drive 100 disclosed in the above Patent reference 1 comprises a rigid circular outer spline 200 with internal teeth, a flexspline 300 positioned inside the circular spline 200 and including external teeth, and an elliptical-shaped wave generator 400 fitted into the flexspline 300 through a bearing. The flexspline 300 includes a cylindrical portion 310, an annular diaphragm portion 320 continuously extending from the proximal side of the cylindrical portion 310, a boss (attachment portion) 330 formed in the center of and integrally with the diaphragm portion 320, and external teeth 340 formed on the outer peripheral surface of an open end portion which is positioned at the distal end of the cylindrical portion 310.
When the wave generator 400 is rotated by the rotation of the motor rotation shaft of the drive source, the flexspline 300 undergoes elastic deformation together with the outer race of the bearing which allows elastic deformation, so that the meshing points between the external teeth 340 of the flexspline 300 and the internal teeth of the rigid circular outer spline 200 displace in the circumferential direction. In general, since the rigid circular outer spline 200 is fixed to a stationary member, a rotation force is output from the boss (attachment portion) 330 of the flexspline 300 while the speed is greatly reduced in accordance with the difference of the number of teeth between the external teeth and the internal teeth.
In the flexspline 300, the cylindrical portion 310 which allows elastic deformation and the annular diaphragm portion 320 are joined together via a bent portion extending from the proximal end of the cylindrical portion 310 in the direction radially and inward toward the center of the boss 330. The flexspline 300 includes the annular boss (attachment portion) 330 integral with and positioned at the inner periphery of the diaphragm portion 320, and the external teeth 340 formed on the outer peripheral surface of the open end portion which is positioned at the distal end of the cylindrical portion 310.
This conventional harmonic gear drive 100 is light-weighted and has no backlash when compared with other reduction gear drives such as an involute planetary gear drive, and a cycloid planetary gear drive, and therefore it is suitable for applications where high positional accuracy is required.
For applications to products such as a robot where silence is required, there is a demand on the harmonic gear drive 100 disclosed in Patent reference 1 to reduce noise. However, the harmonic gear drive 100 has a structure which indispensably causes noise.
To be more specific, the harmonic gear drive 100 does not allow clearance between the flexspline 300 and the wave generator 400 in terms of its structure, and therefore the wave generator is lightly press-fitted into the flexspline 300 allowing the flexspline to be deformed into an elliptical shape.
However, since the outer peripheral surface of the wave generator 400 is elliptical, the inner peripheral surface of the flexspline 300 changes its deformed amount at each point by the rotation of the wave generator 400. Therefore, each of the contact parts between the flexspline 300 and the wave generator 400 does not provide a complete surface contact, and as shown in FIGS. 10A to 10C, the state of each contact part changes during one rotation of the wave generator 400. During this time, slippage occurs at the contact part, so that sliding contact noise due to metallic materials of the flexspline 300 and the wave generator 400 occurs in accordance with the rotational frequency upon high-speed rotation of the wave generator 400.
If the harmonic gear drive 100 is disassembled after use, irrespective of the used time thereof, marks of slippage can be found at the contact parts between the flexspline 300 and the wave generator 400. Further, in the case of the harmonic gear drive 100 which is used for an extended period of time, a fretting corrosion may occur due to this slippage, which leads to a decrease in the service life of the product.
In view of the above, the present invention seeks to provide a harmonic gear drive which can restrict a noise due to slippage as well as a decrease in the service life of the product.